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In the winter of 1967, Jocelyn Bell Burnell pored over the near-frozen dials of a radio telescope. Between curses, she breathed on the instruments hoping to thaw them when, suddenly, the telescope’s recording chart sputtered to life and began transmitting a series of regularly spaced ticks.
This was the second time Bell Burnell had observed the puzzling metronomic space signals as a doctoral student working with the Cambridge astronomer Antony Hewish. Initially unsure what could cause such a measured celestial blink, Bell Burnell and her colleagues jokingly called the beating emissions “LGM” for Little Green Men.
The second time the telescope picked up a similar signal, she knew it wasn’t a quirk in the equipment or an extraterrestrial invitation. Bell Burnett had discovered pulsars—and astrophysics would never be the same.
In 1974, however, it was Antony Hewish whose “decisive role in the discovery of pulsars” would be honored with a Nobel Prize. In later years, Hewish would diminish, with defensive bluster, Bell Burnell’s contribution. “It’s a bit like an analogy I make — who discovered America? Was it Columbus or was it the lookout? Her contribution was very useful, but it wasn’t creative,” Hewish told interviewers in 2007.
But Bell Burnell was always more than a lookout. Susan Jocelyn Bell was born in Northern Ireland in 1943 and encouraged by her parents to pursue a clear propensity for understanding things. She and her family protested fiercely when, on the first Wednesday of secondary school, the girls were segregated for training in the art of “domestic science,” while their male peers pored over Bunsen burners and beakers.
She went on to study at the University of Glasgow, where she again found herself defined by her gender rather than her brain. For two years, whenever Bell Burnell entered a lecture hall her male peers whooped, cat-called, and banged their desks. “It was a little isolating. I had to work very much on my own,” she recalled during a TEDx talk in 2013.
After enduring years of the simian ritual, Bell Burnell made haste for Cambridge in 1965 to pursue a PhD studying under the radio astronomer Antony Hewish. Clad in cat-eye glasses, she spent two years constructing a radio telescope of Hewish’s design — a four-acre affair consisting of wires and pylons with galactic radiation receptors. This vineyard-like tessellation was originally built to study quasars — scintillating deep-space objects discovered in the early 1960s.
The first time the telescope’s radio-frequency needle recorded a regularly timed radiation signal, the team was convinced a glitch had befallen their equipment. What aside from human interference or some intelligent messenger could account for the clockwork pulses of energy? The Cambridge researchers were plagued by the Little Green Men mystery for weeks until Bell Burnell detected a second — and later a third and fourth — percussive signal from separate corners of the heavens.
As the probability of detecting multiple galactic dispatches from distant, intelligent civilizations was near zero, the scientists sought a solution consistent with the laws of physics and the scope of the universe. Hewish interpreted the data as the result of neutron stars or pulsars: superdense dead stars that emit radiation from their magnetic poles like strobe lights.
Before Bell Burnell divined the cosmic transmissions, it was believed that when stars died they simply exploded, releasing their energy in volatile displays we call supernovae. But her discovery suggested that a supernova may not lead to the wholesale destruction of a star — that something might stick around. Pulsars, Hewish and Bell Burnell would establish, were the neutron-rich cores of dead stars emitting radio waves as they rotated around a highly magnetized axis. Pandora’s box was open to all sorts of stellar post-mortem possibilities, most notably the theories of a young astrophysicist named Stephen Hawking, whose ravings about black holes were suddenly taken seriously.

Burnell’s pulsar discovery changed the field of astrophysics forever. SXP 1062 (right) is between 10,000 and 40,000 years old. (NASA)

Bell Burnell would go on to receive her PhD in 1968, sans Nobel, despite co-authoring the article in Nature that would lead to Hewish’s nomination. But it wasn’t just the Nobel committee in Stockholm who were guilty of a double standard. Following the discovery of pulsars, Bell Burnell faced casual sexism from the media and public as well.
“When the press found out I was a woman, we were bombarded with inquiries,” she said. “My male supervisor was asked the astrophysical questions while I was the human interest,” she recalled in an interview with the Belfast Telegraph in 2015. “Photographers asked me to unbutton my blouse lower, whilst journalists wanted to know my vital statistics and whether I was taller than Princess Margaret.”
In the years since the discovery of pulsars Bell Burnell has been a vocal critic of the traditional white male power structure that dominates Western scientific thought and academia. When she was appointed the chair of the physics department at Open University in 1991, Bell Burnell was one of only two female physics professors in the U.K. “Throughout my working life, I’ve been either one of very few women or the most senior woman in the place,” she told the TEDx audience.
After obtaining her PhD, Bell Burnell worked part time for many years while raising a family and following the career of a “peripatetic” husband. “I am very conscious that having worked part time, having had a rather disrupted career, my research record is a good deal patchier than any man’s of a comparable age,” she said in a 1996 interview with the Institute of Physics.
Still, Bell Burnell has continued to advance, earning visiting professorships at Oxford and Princeton. She is currently the president of the Royal Society of Edinburgh, Scotland’s national academy of science and the arts.
In public forums, she often repeats the fundamental truth so many people fail to grasp: that the small number of women in STEM in the West is the result of social restrictions and expectations. “The limiting factor,” she points out, “is culture, not women’s brains, and I regret that its still necessary to say that.”

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Cosmic rays — fast-moving, high-energy nuclei — pervade the Universe. We know that the lower-energy variety that we detect on Earth is funnelled by the solar wind. However, higher-energy cosmic rays have an isotropic distribution due to scattering that makes it difficult to identify their source, although they are likely to be generated by high-energy phenomena like supernova explosions and jets from active galactic nuclei. By looking at the ultrahigh-energy end of the cosmic ray spectrum (on the order of exa-electron volts and higher, where cosmic rays are not scattered by solar-scale magnetic fields), the Pierre Auger Collaboration detected an anisotropy in their arrival directions that indicates an extragalactic origin.
Ultrahigh-energy cosmic rays are rare: typically one cosmic ray with an energy > 10 EeV hits each square kilometre of the Earth’s surface per year. The Pierre Auger Observatory in Argentina detects cosmic rays using two combined techniques: telescopes to detect fluorescence from cosmic-ray-generated air showers, and a network of 12-tonne containers of ultrapure water, spread over an area of 3,000 square kilometres. Photomultiplier detectors in the containers observe the faint Cherenkov radiation generated when cosmic-ray-generated muons encounter water molecules. By reconstructing the cone of emission of the muon (analogous to an aircraft’s sonic boom) an incident direction can be derived. By analysing 32,187 cosmic rays detected over 12.75 years, a map of the sky was produced (pictured), showing evidence of an enhancement (5.2 σ significance) in a region away from the Galactic Centre (marked with an asterisk; the dashed line indicates the Galactic plane). The distance of this hotspot from the Galactic Centre (~125°) points towards an extragalactic origin of ultrahigh-energy cosmic rays, reinforcing previous (less conclusive) results from the Collaboration at lower energies.

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Four Eyes of Tatooine by Stefan Lines (created whilst working on his PhD thesis in Dr Leinhardt’s Planet Formation Group at the University of Bristol). This computer generated image, based on data taken from actual super-computer simulations, shows the formation of a planet around a binary star — a so called ‘circumbinary planet’. Tiny unit vectors show the magnitude (colour) and direction (orientation) of the acceleration of millions of tiny rocky ‘planetesimals’ that eventually coalesce to form a planet.

Why do you think space inspires people so much?

I guess that it’s our human nature to explore and we see it, at least most of us can see it, at night and I think that it’s in our nature to ask why things look the way they do, or why a process happens. And since you can look up in the sky and see a bunch of lights, it’s natural to question what that is and want to be able to explain it and go there. So I think it’s just our natural instinct to want to explain what we don’t understand, especially if we can see it.

Investigating the overall brightness of planets (and moons) provides insights into their envelopes and energy budgets. Phase curves (a representation of the overall brightness versus the Sun–object–observer phase angle) for Titan have been published over a limited range of phase angles and spectral passbands. Such information has been key to the study of the stratification, microphysics and aggregate nature of Titan’s atmospheric haze and has complemented the spatially resolved observations showing that the haze scatters efficiently in the forward direction. Here, we present Cassini Imaging Science Subsystem whole-disk brightness measurements of Titan from ultraviolet to near-infrared wavelengths. The observations show that Titan’s twilight (loosely defined as the view at phase angles ≳150°) outshines its daylight at various wavelengths. From the match between measurements and models, we show that at even larger phase angles, the back-illuminated moon will appear much brighter than when fully illuminated. This behaviour is unique in our Solar System to Titan and is caused by its extended atmosphere and the efficient forward scattering of sunlight by its atmospheric haze. We infer a solar energy deposition rate (for a solar constant of 14.9 W m−2) of (2.84 ± 0.11) × 1014 W, consistent to within one to two standard deviations with Titan’s time-varying thermal emission from 2007 to 2013. We propose that a forward scattering signature may also occur at large phase angles in the brightness of exoplanets with extended hazy atmospheres and that this signature has a valuable diagnostic potential for atmospheric characterization.

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Analyses of images taken by the Rosetta spacecraft reveal the complex landscape of a comet in rich detail. Close-up views of the surface indicate that some dust jets are being emitted from active pits undergoing sublimation.
When do 18 holes not make for a pleasant afternoon playing golf? When the 18 holes are located on the surface of a comet speeding through the Solar System. Vincent et al.(1) describe the holes, also called pits, that comprise one of the many discoveries of the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P). The Rosetta spacecraft went into orbit around 67P in August 2014, and the surprises have been coming fast since then. Vincent et al. propose a mechanism for the formation of the pits and identify them as one of the sources of active dust jets.
Comets are the most primitive bodies in the Solar System; they are the remnants of its formation process. Comets therefore retain a physical and chemical record of the conditions and materials in the solar nebula — the gas and dust cloud out of which the Sun and planets formed 4.56 billion years ago. Conveniently, comets have spent most of that time in two very cold storage locations: the Kuiper belt beyond the orbit of Neptune and the spherical Oort cloud outside the planetary region, stretching halfway to the nearest stars. The distant Oort cloud is the source of the long-period comets that have orbital periods ranging up to millions of years. The Kuiper belt is the source of the Jupiter-family comets, such as 67P, which typically have periods of less than 20 years and orbital dynamics that are strongly affected by Jupiter.
As a comet approaches the Sun and warms up, the central solid part, known as the cometary nucleus (comprised of volatile ices and primitive meteoritic material), begins to sublimate and becomes enveloped by a freely outflowing atmosphere called the coma. One of the first surprises for Rosetta, the first ever comet-rendezvous mission, was the odd shape of the target comet’s nucleus (Fig. 1a)(2). Although some nuclei comprised of two large pieces and looking like a bowling pin had been observed before by fly-by missions to other comets, the two lobes of 67P sit on top of each other, with a narrow ‘neck’ in between. There is intense speculation as to how this odd configuration may have formed. Did two cometary nuclei gently collide randomly in the solar nebula, or is the nucleus a single piece that has been oddly sculpted by sublimation processes? Although the former is the more likely scenario, some scientists on the mission suspect the latter.

Vincent et al.(1) analysed images of comet 67P taken by the Optical, Spectroscopic and Infrared Remote Imaging System cameras on the Rosetta spacecraft. a, The complex nucleus topography includes large, flat-floored basins (indicated by white arrows). A large, circular pit is visible just above the centre of the image (red arrow). b, A string of pits dot the surface of the cometary nucleus. In active pits such as these, bright jets of dust are seen being emitted from the sunlit walls. The contrast of this image has been enhanced to highlight the interiors of the pits and the jets. As a result, the cometary surface looks very bright, but in reality it reflects only about 6% of the incoming sunlight — roughly the same as the black toner particles in a laser printer cartridge.ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Rosetta’s camera system, the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS), is comprised of narrow-angle and wide-angle digital cameras. As the OSIRIS team of scientists2 began to map the surface of the nucleus using the cameras, they discovered 18 pits on the surface, which Vincent et al. now describe more thoroughly. The cometary nucleus has a diameter of approximately 4 kilometres. The pits are typically about 200 metres in diameter and about 180 metres deep. Pit-like features have been observed on other cometary nuclei, but the morphology of the pits on 67P has not been seen before. They typically have cylindrical shapes with circular openings and near-vertical walls (although at least one pit seems to be lying at a steep angle). And some of the pits are clearly active: images of pits that are illuminated by sunlight show dust jets emanating from their walls and/or floors (Fig. 1b).
How did the pits form? Vincent et al. suggest that they are ‘sink holes’, which formed when material near the surface of the nucleus collapsed into the low-density interior. Rosetta’s Radio Science Investigation team has found(2) that the nucleus has an average bulk density of only 470 ± 45 kilograms per cubic metre, about half the density of solid water ice. But the Grain Impact Analyser and Dust Accumulator instrument has measured(3) a dust-to-ice mass ratio of 4 ± 2, suggesting that silicates and organics, rather than ices, make up about 80% of the mass of the nucleus. This in turn implies that 75–85% of the nucleus interior is empty space, a parameter known as porosity. A high porosity is predicted by the leading scenarios for the internal structure of cometary nuclei, which suggest that they are aggregates(4) of smaller, icy bodies that gently came together in the solar nebula. These aggregates are also referred to as rubble piles(5). This concept has provided insights into the behaviour of comets, such as random and other splitting events.
The morphology of 67P’s surface is dominated in some areas by large, flat-floored basins, similar to features seen on the nucleus of comet(6) Wild 2. It has been suggested that these are sublimation basins that slowly widen as the walls sublimate, leaving large, non-volatile particles that cover the basin floor. The basins cannot be impact craters because they have the wrong size distribution (there are too many large ones), and because not many impact craters are expected on a small cometary nucleus such as 67P.
Could the pits described by Vincent et al. be the precursors of the basins, slowly widening as their walls sublimate? Many of the pits found by OSIRIS are located in the same region on the nucleus where many of the large sublimation basins are found. Both comet 67P and comet Wild 2 are relatively young — that is, they have only recently (within the past 60 years) been perturbed by the gravitational field of Jupiter to perihelion distances (the point in their orbit closest to the Sun) at which it is warm enough for water ice in the nucleus to sublimate, and at which the activity that manifests itself as the bright cometary coma and tails begins. If this is so, why are sublimation basins not observed on other, perhaps older, Jupiter-family comets such as Tempel 1 and Hartley 2? Older nuclei may have accumulated thicker layers of non-volatile materials that have buried the sublimation basins and substantially lowered the activity levels of those comets.
Rosetta has already indicated that it has more surprises for us. On 13 June 2015, the orbiter began receiving signals from the Philae lander, which is on the surface of the comet nucleus and was last heard from in November 2014. With its batteries recharging, Philae probably has much more information to transmit about its final landing location. Also, the activity of the nucleus is expected to reach a maximum soon after the comet passes through perihelion at 1.25 astronomical units from the Sun (a point about 25% farther from the Sun than Earth’s orbit) in mid-August 2015. Rosetta will then follow 67P away from the Sun as cometary activity begins to wane. What changes will we see on the nucleus surface? And how will this alien golf course look from Rosetta’s vantage point then?

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Astronomers have discovered more than 850 faint galaxies in a galaxy cluster that could be made mostly of dark matter.
Using archived images from the Subaru Telescope in Hawaii, a team led by Jin Koda at Stony Brook University in New York searched for observations of the Coma galaxy cluster, which is roughly 101 million parsecs (330 million light years) away. The team found 854 ultra-diffuse galaxies, a class of faint galaxy that can be as large as the Milky Way, but which has only 0.1% the number of stars. For these galaxies to remain gravitationally bound together, the researchers show that more than 99% of their mass must be dark matter.
This suggests that the crowded environment sucks gas away from these galaxies, leaving them largely unable to form stars.

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Observations of galaxies that formed early in the Universe’s history reveal much lower dust levels than are found in sources from a slightly later era. It seems that galaxies underwent rapid change during a relatively short period.
The study of the most distant galaxies, observed as they were about 1 billion years after the Big Bang, is crucial for our understanding of the star-forming activity and the physical processes at work in these young systems. Capak et al.(1) present a study of nine such galaxies using a linked-up telescope array. They find that the dust and gas properties in these systems hint at an interstellar medium (ISM) that is much less evolved than in galaxies about 2 billion years older. This suggests that there was a rapid change in the overall properties of galaxies during the early life of the Universe.
The expansion of the Universe shifts the ultraviolet (UV) light emitted by newly formed stars in remote systems to longer (visible and near-infrared) wavelengths that, unlike UV light, can be observed by ground-based telescopes. The most distant objects known today are detected as a result of a break in the continuum of their redshifted spectra at wavelengths of around 0.1 micrometres; this is due to the absorption of UV photons by neutral hydrogen in the intergalactic medium. The absorption occurs for photons with energies corresponding to wavelengths shorter than the Lyman-α line of hydrogen (1,216 nm), and galaxies whose distances have been estimated by this method are known as Lyman break galaxies (LBGs). The most comprehensive surveys undertaken so far have led to detections of very young LBGs that formed approximately 0.5 billion years after the Big Bang(2).
The presence of interstellar dust complicates the study of galaxies, and affects measurements of fundamental, observationally derived properties such as the star-formation rate. This is because dust is efficient at absorbing the energetic UV photons (a proxy for the star-formation rate) that are emitted by young stars and at re-emitting their energy in the infrared domain, at wavelengths longer than 5 μm. This is a complex process that depends not only on the amount of dust present, but also on its distribution relative to the stars and on its composition(3). Overall, dust substantially reduces the intensity of stellar light reaching the telescopes(4).
A straightforward method to account for the UV light produced in galaxies involves observing the radiant energy that is absorbed by dust, is re-emitted and is then redshifted in the far-infrared and submillimetre domains. However, the low sensitivity of detectors, combined with the poor spatial resolution achieved by single-dish telescopes, make surveys of high-redshift galaxies at these wavelengths less efficient than those at optical or near-infrared wavelengths. Even the Herschel Space Observatory, which detected(5) the infrared emission from dust in galaxies at redshifts of up to 2–3 (corresponding to a time roughly 2 billion to 3 billion years after the Big Bang), was able to detect only hyper-luminous sources at much larger distances(6).
Given that directly measuring the long-wavelength emission from dust is so challenging, astronomers resort to empirical relations to derive dust’s infrared luminosity. One such relation links this luminosity to the stellar UV luminosity(7) for a representative sample of nearby, actively star-forming galaxies, for which both luminosities have been accurately measured. Unfortunately, this recipe is not universally applicable because it depends on the properties of the ISM (such as the composition of dust and its distribution relative to the stars), as well as on the stellar populations in the galaxies(8). Despite these caveats, however, it is extensively used to estimate the level of obscuration of the stellar UV light by ISM dust for galaxies across a wide redshift range. It will therefore be important to check the validity of this relationship — especially for young, high-redshift systems. A critical evaluation could also yield clues to the properties of the ISM at those early times.
Capak et al. used the Atacama Large Millimetre Array (ALMA), which was designed to overcome both the resolution and sensitivity problems (Fig. 1). Being an interferometer (a series of telescopes linked up to combine astronomical observations), ALMA has a small field of view that is suitable for observing well-centred sources, and it can detect the weak submillimetre emission originating from dust in ordinary galaxies at high redshifts. The authors used 20 of ALMA’s antennas in unison to observe the dust and gas emissions of 9 typical LBGs located at redshifts 5–6; these correspond to a time when the Universe was about 1 billion years old.

Capak et al.(1) used 20 of ALMA’s antennas to study the interstellar medium (ISM) of 9 galaxies that were present when the Universe was only about 1 billion years old. The authors found that their sources contain a smaller amount of dust than expected. Some of the galaxies in the sample may have an ISM similar to that of the Small Magellanic Cloud (a satellite galaxy of the Milky Way), which is visible here (right of centre) as the smaller of the two Magellanic Clouds above the antennas.

Capak and colleagues selected their sample from the Cosmic Evolution Survey field, a two-square-degree area that has been extensively observed by most of the major telescopes, from the ground and from space. ALMA detected the thermal dust emission in four galaxies, and an ISM spectral line emitted from gaseous carbon at a wavelength of 158 μm in all nine of them; the carbon feature is the dominant ISM emission line of galaxies in the far-infrared domain. Such a high detection rate is outstanding, because previous attempts failed to simultaneously detect the carbon feature and thermal dust emission(9).
The authors’ study argues for a very low dust content and stellar-light obscuration in these systems. The four galaxies whose thermal dust emission was detected may harbour an ISM similar to that of the Small Magellanic Cloud (a satellite galaxy of the Milky Way), which is characterized by a low abundance of elements heavier than helium. The upper limits put on the dust emission of the remaining five sources call for an even more extreme situation with a much lower infrared emission. That seems to be at odds with the observed UV luminosity of these systems. The enhanced carbon emission-line intensities also suggest low dust levels relative to the gas present in these early galaxies, although other explanations cannot be excluded.
An immediate consequence of these findings is that the classical calculations used to derive obscuration due to dust from the observed UV continuum luminosity are unlikely to be valid for LBGs in the early Universe. As a result, the star-formation rate considered to be appropriate for these galaxy types is likely to have been overestimated by factors of between two and four in previous studies. Last but not least, this pioneering work paves the way for future observational campaigns. Although observing the low levels of dust emission from large samples of high-redshift galaxies may prove challenging even for ALMA, Capak and co-workers’ finding of enhanced carbon emission lines should become a useful tool in the study of star-forming galaxies at those early epochs.

Planets orbiting a binary star system — like Tatooine, the fictional home planet of Luke Skywalker in Star Wars — could form with surprising ease.
Most known circumbinary planets orbit close to their stars, where the competing gravitational forces from the two stars make the orbits of nearby objects unstable or intersect. This prevents debris from clumping together to form planets. But Benjamin Bromley of the University of Utah in Salt Lake City and Scott Kenyon of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, show with simulations that a zone exists near the host stars where the orbits of debris wobble, but do not cross, allowing for planet formation.
This suggests that Earth-sized ‘Tatooines’ could be common and that more are likely to be discovered soon.

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Astronomers have seen their best glimpse yet of stars forming in the early Universe.
The ALMA radio telescope in Chile explored the SDP.81 galaxy, which is 3.6 billion parsecs (11.7 billion light years) away from Earth. Its light was magnified and distorted by the gravitational pull of another galaxy between it and Earth, but a model developed by Yoichi Tamura of the University of Tokyo and his team corrected the distortions. Their images reveal many cold clouds of dust and gas that are driving a rapid rate of star formation.
Several research teams have analysed the ALMA data to characterize other aspects of this galaxy.

(a) ALMA 3-color image of SDP.81 (1.0, 1.15 and 1.3 mm for blue, green and red, respectively) overlaid with the Hubble WFC3/F160W (1.6 µm) image where the stellar light of SDSS J0903 is subtracted (contours). Two sets of counter-images of stellar peaks are indicated by filled and open stars, respectively. The synthesized beam size is indicated at the bottom-left corner. The origin of the image is taken at the position of a central compact non-thermal source. (b) The ALMA 1.0 mm image. Upper inset shows CO (5–4) spectra at the positions of the source A (upper) and E (lower). Bottom inset shows the spectral energy distribution of the central compact source, which is well fitted by a power-law function with a spectral index of −0.64 (solid line), suggesting the synchrotron emission. (c) The modeled brightness distribution on the image plane. The image is smoothed by a Gaussian with FWHM = 23 mas. The inner and outer ellipses represent radial and tangential critical curves, respectively. (d) The modeled brightness distribution on the source plane, which is 0.5′′ on a side. The star represents the source position of the stellar peaks denoted as filled stars in (a). The position of this panel is indicated as a dotted square in (c). The solid curves represent the caustics. The scale bar at the bottom-left corner shows a physical scale of 200 pc at z = 3.042.

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Analysis of Kepler data has yielded the smallest known mass for an exoplanet orbiting a normal star. Its mass and size are similar to those of Mars, setting a benchmark for the properties of exoplanets smaller than Earth.
With a mass one-tenth that of Earth’s, half its diameter, and a residual gleam of long-lost habitability, Mars has an outsized hold on our imagination. Jontof-Hutter et al.(1) report the discovery of Kepler-138 b — an alien world that has a mass, a radius and perhaps an overall composition very similar to those of Mars. In contrast to warm Earth or cold Mars, however, this planet is baking hot because of its proximity to its host star. The current exoplanet census is dominated by planets a few times the size of Earth and on orbits close to their stars, and so our Solar System — with its four small inner planets — seems somewhat marginalized. The detection of such a small exoplanet in a tight orbit could help to clarify how we fit into the big picture.
NASA’s successful Kepler space observatory made the observations that enabled the detection of this low-mass world. Between 2009 and 2013, Kepler monitored more than 150,000 stars in a patch of sky just above the mid-plane of the Milky Way, and recorded the periodic diminutions in brightness that occur when planets with appropriately aligned orbits transit (pass in front of) their parent stars(2). At the last count, more than 4,600 candidate planets had been identified in the data returned by the Kepler mission(3). Although the bulk of these prospective worlds have radii 2–4 times that of Earth’s and display a bewildering range of compositions, more than 500 candidates have radii smaller than Earth’s. The race is on to elevate these candidates to confirmed planetary detections(4).
Observations of repeating dips in the brightness of a host star, each dip having identical depth and duration, might seem to constitute fail-safe evidence of an orbiting planet. There is, however, a host of potential false alarms that can compromise a detection, notably the possibility that a background, eclipsing binary star registers on the same detector pixel as the candidate planet’s host star, so that periodic eclipses in the distant system can mimic planetary transits of the target star(5). So far, more than 1,000 of Kepler’s planets have been vetted and confirmed, but the process of follow-up evaluation (especially for the low-mass worlds) will continue for decades.
Transit detections provide an estimate of a planet’s size, but they give no information about its mass. For massive transiting planets, masses can be estimated by Doppler spectroscopic monitoring of the host star’s velocity variations along the line of sight of the observation. This option, however, is unavailable for a planet such as Kepler-138 b, which is too small to induce detectable wobbles in the host star’s motion(6).
During the nineteenth century, Mars presented a similar challenge to astronomers who were trying to estimate its mass. In 1877, the discovery of the planet’s tiny satellites (Phobos and Deimos) allowed the mass of Mars to be determined accurately and directly. Earlier estimates, however, beginning with the efforts of Jean Baptiste Joseph Delambre in 1806, were obtained by carefully accounting for Mars’s gravitational influence on other planets in the Solar System, and calculating their effects on the precisely measurable daily location of the Sun. By 1850, refinements of this technique had narrowed the Martian mass estimate to within 20% of its true value(7).
Jontof-Hutter and colleagues have drawn from the same time-tested notebook of dynamical astronomy to deduce the mass of Kepler-138 b. The planet is part of a system (Fig. 1) that contains two additional transiting planets, Kepler-138 c and Kepler-138 d, which have periods of 13.8 and 23.1 days, respectively, and radii slightly larger than that of Earth. The inner planet pairing between 138 b and 138 c has an orbital-period ratio of nearly 4:3, whereas the outer pairing between 138 c and 138 d has a ratio near 5:3. Because the period ratios are almost exact ratios of integers, this enables the gravitational distortions of the orbit of one planet by the other planets to gradually ebb and flow, producing observable departures from expected strict regularity in the transit times. Measurements of the planets’ transit timing variations constitute a nonlinear, computationally intensive inverse problem, the solution of which has allowed the authors to deduce the masses of all three planets.

The graphic depicts the orbital architecture of an exoplanet system discovered by the Kepler space observatory. Jontof-Hutter et al.(1) report the sizes and masses of planets Kepler-138 b, Kepler-138 c and Kepler-138 d. Kepler-138 b, on a 10.3-day orbit, has the smallest measured mass of any transiting exoplanet — it has a size and mass similar to those of Mars. Kepler-138 c and Kepler-138 d are slightly larger than Earth and have orbital periods of 13.8 days and 23.1 days, respectively. The orbits are drawn relative to the size of Mercury’s 88-day orbit.

The radii of known exoplanets generally display surprisingly little correlation with their masses, especially for planets that are somewhat more massive than Earth. Jontof-Hutter et al. report a mass of around 0.07 Earth masses for Kepler-138 b, imbuing it with a bulk density of about 3 grams per cubic centimetre — consistent with a purely rocky composition roughly akin to that of Mars. The planet’s mass is similar to, but somewhat less than that of Mars (by contrast, its surface temperature is much higher). Whether we are probing an outlier world, or whether the authors’ discovery points to an important trend among low-mass planets — whereby smaller radii exhibit strong positive correlation with smaller masses — is as yet unclear.
The derivation of dynamical properties from transit measurements requires timing data with split-second precision, and so the mass and density of Kepler-138 b both have large uncertainties, of a factor of about two. The prime drivers of the uncertainty are the relatively low number of planetary transits that have been observed and the timing precision. These can be improved both with longer observation campaigns and, eventually, with space missions capable of observing at intervals shorter than Kepler’s 1-minute limit. The European Space Agency’s PLATO (Planetary Transits and Oscillations of Stars) mission planned for the mid-2020s, for example, will be able to monitor the brightest transit-bearing stars at a temporal resolution of 2.5 seconds.
Kepler-138 b orbits a red dwarf star, the mass, radius, chemical composition and space motion of which are close to those of the ‘average’ star in the Milky Way. The Kepler-138 planetary system also seems to be unexceptional when placed in its galactic context. The Kepler mission has demonstrated that it is extremely common for stars to harbour multiple planets with orbital periods of less than 100 days and masses of lower than 20 Earth masses. By contrast, the reaches of the Solar System interior to Mercury, which has an 88-day orbit, are completely empty. It is imperative to improve our understanding of how our system’s architecture and evolution fit into the overall census — the authors’ study is a step towards that goal.
A pressing question is whether planets such as those orbiting Kepler-138 formed in situ, or whether they accumulated in colder, distant regions of the protoplanetary disk before migrating inwards. Short-period, low-mass planets that cannot retain hydrogen atmospheres, and the densities of which are similar to mixtures of rock and ice, present evidence in favour of inward migration from cold regions. This is because planets with an icy bulk cannot have formed close to their star. Additional, increasingly accurate mass and radius measurements, both for Kepler-138 b and for other small exoplanets, will be facilitated soon by NASA’s TESS (Transiting Exoplanet Survey Satellite) mission(8), scheduled for 2017, and will help to provide an answer.